This invention relates to a CO-absorbing solution, and a process for absorbing CO and a process for recovering CO, and more particularly to an absorbing solution for absorbing CO from a CO-containing gas and a process for absorbing CO from a CO-containing gas with the absorbing solution and a process for recovering CO from the CO-absorbed solution.
Some of waste gases from the iron works and the petrochemical industry contains much CO (carbon monoxide) and CO, if effectively recovered from the waste gases, can be used as a raw material for the C.sub.1 chemistry now regarded as a promising field.
CO can be separated or concentrated according to a cryogenic separation process, a solution absorption process or an adsorption process (PSA process). The cryogenic process provides separation of CO, based on cooling to -65.degree. to -210.degree. C. to liquefy CO. However, when the CO-containing gas also contains much nitrogen, whose boiling point is approximate to that of CO, together, it is difficult to separate CO from nitrogen. Furthermore, the cryogenic separation process requires a low temperature and a high pressure for the liquefaction of CO, making the lieuqfaction facility expensive. Thus, the CO separation and recovery from a CO-containing gas are now principally based on the solution absorption process and the adsorption process.
The adsorption process is very economical in case of a small scale or medium scale CO separation apparatus, and not in case of a large scale apparatus. Furthermore, in order to recover a CO gas of high purity, adsorption and desorption must be repeated. That is, the solution absorption process is suitable only for a large scale recovery of a CO gas of high purity.
An absorbing solution capable of absorbing only CO is necessary for the separation and recovery of CO from a CO-containing gas, and typical CO-absorbing solution now available include aqueous CO-absorbing solutions and non-aqueous CO-absorbing solution, and the solution absorption processes now available are identified with the CO-absorbing solutions used.
Typical solution absorption processes include a cuprous solution process, a COSORB process [D. J. Haase, D. G. Walker : Chem. Eng. Prog. 70 (5), 74 (1974)]and a HISORB process [Kagakukogyo, January issue (1986), pp. 66-69 ].
The cuprous solution process is based on use of an aqueous ammonia solution of cuprous formate as an CO-absorbing solution, where CO absorption is carried out at 20.degree. C. under a high pressure such as 150-200 atm.
The COSORB process is based on use of a toluene solution of cuprous tetrachloroaluminate, where CO absorption is carried out at about 40.degree. C. under 64 atm. However, the CO-absorbing solution reacts with water and thus the water content of a CO-containing gas must be kept to less than 1 ppm.
The HISORB process is based on use of an organic solvent absorbing solution comprising a complex compound of hexametaphosphateamine (hmpa), which is also called "tris (dimethylamino) phosphine oxide", with cuprous chloride (CuCl) and toluene as a CO-abosrbing solution, where CO absorption is carried out at the ordinary temperature under a pressure of 1 to 18 atm. The CO-absorbing solution has a characteristic of absorbing and dissolving CO in the form of a metal complex in a low temperature region and evolving the absorbed CO at an increased temperature, and thus has been regarded as 15 a very effective CO-absorbing solution [Japanese Patent Application Kokai (Laid-Open) Nos. 56-118720, 57-19013, etc]. That is, the HISORB process based on use of an organic solvent CO-absorbing solution of hexametaphosphateamine-CuCl-toluene has a very high CO recovery such as 95% or more.
However, in the HISORB process, the corrosiveness of the organic solvent solution on materials of construction of the apparatus has not thoroughly taken into consideration. For example, vigorous general corrosion takes place on the entire surface of carbon steel or low alloy steel of up to 5% Cr steel, and pitting corrosion takes place considerable on ferritic stainless steel of 19% Cr--2% Mo and austenitic stainless steel of 18% Cr--8% Ni (typical grades: SUS 304 and SUS 316), as shown in Table 1.
TABLE 1 __________________________________________________________________________ Corrosion in HISORB absorting solution Corrosion Corrosion depth atmosphere Material (mm) Corrosion form __________________________________________________________________________ 5 --M .multidot. hmpa + Carbon steel 1.25 Entire surface corrosion 2 --M .multidot. CuCl + 21/4 Cr-1 Mo steel 1.10 " 1 --M .multidot. toluene 5 Cr steel 1.03 " 95.degree. C./100 hr 13 Cr steel 0.62 Pitting corrosion + entire surface corrosion 17 Cr steel 0.38 Pitting corrosion 19 Cr 2 Mo 0.21 " SUS 304L* 0.30 " SUS 316L* 0.13 " DP-3* 0.09 " Inconel 825* 0.06 " Inconel 625* 0.00 No corrosion Hastelloy C* 0.00 " Titanium 0.00 " Teflon 0.00 " Al.sub.2 O.sub.3 ceramics 0.00 " __________________________________________________________________________ *Major components SUS 304L: 10Ni, 18Cr SUS 316L: 12Ni, 18Cr, 2Mo DP3: 7Ni, 25Cr, 3Mo Inconel 825: 44Ni, 22Cr, 3Mo, 2Cu Inconel 625: 60Ni, 22Cr, 9Mo, 3Nb Hastelloy C: 58Ni, 16Cr, L6Mo (twophase stainless steel)
As a result of extensive studies, the present inventors have found that the corrosion takes place due to a very small amount of water-soluble impurities contained in the organic solvent solution, and the impurities include HCl and acidic chlorides. Hexametaphosphateamine and toluene are organic substances of non-aqueous solution and have no corrosiveness Furthermore, a corrosive chemical CuCl as dissolved in the hexametaphosphateamine takes a chelate form and shows an immunity to corrosion.
As shown in Table 1, materials of construction having a corrosion resistance to the organic solvent solution includes Ni-based stainless steel species of high Ni (&gt;50%), high Cr (&gt;15%) and high Mo (&gt;5%) such as Hastelloy C and Inconel 625, highly corrosion-resistance materials such as titanium (Ti), zirconium (Zr), etc.; ceramics; and polyfluorocarbon resins such as Teflon, etc. These materials are very expensive, as compared with stainless steel, and particularly ceramics, Teflon, etc. cannot be used as materials of construction for columns, heat exchangers, pumps, etc. from the viewpoint of mechanical strength.